The viability of carbon capture sequestration (CCS) is dependent on the secure storage of CO_{2} in subsurface geologic formations. Geomechanical failure of caprock is one of the main reasons of CO_{2} leakage from the storage formations. Through comprehensive assessment on the petrophysical and geomechanical heterogeneities of caprock, it is possible to predict the risk of unexpected caprock failure. To describe the fracture reactivation, the modified Barton–Bandis model is applied. In order to generate hydrogeomechanically heterogeneous fields, the negative correlation between porosity and Young’s modulus/Poisson’s ratio is applied. In comparison with the homogeneous model, effects of heterogeneity are examined in terms of vertical deformation and the amount of leaked CO_{2}. To compare the effects of heterogeneity, heterogeneous models for both geomechanical and petrophysical properties in coupled simulation are designed. After 10year injection with petrophysically heterogeneous and geomechanically homogeneous caprock, CO_{2} leakage is larger than that of the homogeneous model. In contrast, heterogeneity of geomechanical properties is shown to mitigate additional escape of CO_{2}. Vertical displacement of every heterogeneous model is larger than homogeneous model. The model with compressive tectonic stress shows much more stable trapping with heterogeneous caprock, but there is possibility of rapid leakage after homogeneous caprock failure.
CO_{2} sequestration in aquifer is an effective and a verified method to reduce the atmospheric CO_{2} airborne fraction. The world’s first commercial approach of CO_{2} storage project is attempted at the Sleipner gas field in the North Sea, and CO_{2} sequestration in a saline aquifer has been regarded as the feasible technology [
To ensure the feasibility of potential storage site, intensive estimation of formation properties should be preceded [
For a much more accurate evaluation of geomechanical change, the coupled mechanism of geomechanical and petrophysical properties was considered. Cappa and Rutqvist [
In this study, we constructed a CCS model with heterogeneous caprock and compared the flow and geomechanical responses to CO_{2} injection with particular focus on the risks to storage security posed by geomechanical deformation. The correlations between geomechanical and petrophysical properties were regarded as a function of porosity, and several numbers of numerical simulations were conducted to evaluate the effects of the heterogeneous caprock during CCS. The amount of stored CO_{2} and vertical deformation of caprock within a twodimensional formation models were compared for several rock properties including parameters such as Poisson’s ratio, Young’s modulus, porosity, and permeability. These properties were distributed in randomly generated heterogeneity conditions. The effects of heterogeneity on caprock were revealed by comparing the average values of heterogeneous model simulation results with homogeneous caprock model. Considering correlation between geomechanical and petrophysical properties can help assessing formation stability and mitigating geological risk. Additionally, insitu tectonic stress is known to play an important role in fracture propagation [
The overburden stress, known as the total stress, induced by external loading, is sustained by the rock matrix and the formation fluid pressure. To evaluate the fluid pressure required to fracture reactivation in caprock, Terzaghi [
The tensile failure potential is affected by applying the assumption that tensile fracture propagation can occur when the fluid pressure exceeds the summation of least principal effective stress and tensile strength [
A hydromechanical fracture permeability model was based on the modified Barton–Bandis model, as shown in Figure
Modified Barton–Bandis model for the change in fracture permeability.
Walsh and Brace [
To analyse the petrophysical and geomechanical properties, sedimentary rocks were studied using samples cored from several wells from the KrishnaGodavari basin on the east coast of India [
Linear relationship and correlation coefficient between petrophysical and geomechanical properties of cores samples from wells in the KrishnaGodavari basin. (a) Young’s modulus and uniaxial compressive strength. (b) Uniaxial compressive strength and effective porosity.
Crossplots of Young's modulus and Poisson’s ratio, showing the degree of clay. Mineralogical substitution occurs between (a) quartz and clay and (b) dolomite and clay.
GEM was used to assess the fracture propagation during the CO_{2} injection, which is a compositional simulator developed by the Computer Modeling Group [
Hydraulic and geomechanical properties for homogeneous model.
Properties  Overburden rock  Caprock  Aquifer 

Permeability (md)  25  0.0001  100 
Porosity  0.21  0.01  0.25 
Rock density (kg/m^{3})  2,260  2,260  2,260 
Young’s modulus, 
5  5  5 
Poisson’s ratio, 
0.25  0.285  0.25 
Maximum fracture permeability, 
—  35  — 
Minimum fracture permeability, 
—  5  — 
In order to assess the effects of the geomechanical rock properties of the fracture reactivation, several simulations were conducted on twodimensional crosssectional model. The model considers permeability and porosity of rock matrix, Poisson’s ratio, and Young’s modulus, which are regarded to be the most important properties for the investigation of the geomechanical state. The formation model was constructed, as shown in Figure
Model description: aquifer, caprock, and overburden. The injector is perforated in bottom three layers of the aquifer.
CO_{2} component properties.
Property  Value 


72.8 

304.2 
Molecular weight  44.01 
Porositypermeability relationship is calculated using a power law. To generate geostatistical hydrogeomechanically heterogeneous fields, the negative correlation between porosity and Young’s modulus is applied. Poisson’s ratio is negatively related to porosity, too. Table
Physical data of geological seals considered for saline CO_{2} sequestration in the Paradox Basin.
Parameter  Value 

Depth (m)  1,707–1,768 
Thickness (m)  9–12 
Porosity (%)  1.6–4.1 
Vertical permeability, 
39.16–36.86 
Horizontal permeability, 
44.32–44.12 
Compressive strength, 
166–311 
Poisson’s ratio, 
0.24–0.33 
Young’s modulus, 
38,562–56,613 
Basin areal extent (km^{2})  28,490 
In this study, a stochastic simulation method is used for heterogeneous reservoir model. To describe formation heterogeneity, Dykstra–Parsons coefficient (
Numerical attributes considered for caprock permeability.

Mean (md)  Variance 

0.3  0.0001  0.25 
0.5  0.0001  1 
0.7  0.0001  3 
To evaluate how heterogeneity considerations for petrophysical and geomechanical properties affect output results, four cases are designed as below:
Case A: heterogeneous petrophysical and homogeneous geomechanical properties
Case B: homogeneous petrophysical and heterogeneous geomechanical properties
Case C: heterogeneous petrophysical and geomechanical properties
Case D: homogeneous petrophysical and geomechanical properties
The effects of heterogeneity are revealed in comparison to homogeneous model. The results are compared in terms of vertical deformation and the amount of leaked CO_{2}.
Figure
Gas saturation of single realization for
Cumulative inplace amounts of CO_{2} in aquifer at different times: Cases A, B, and C.
In order to assess the range on uncertainty, a boxwhisker plot format is used to illustrate minimum, 25th, 50th, 75th percentiles, and maximum values for sequestered CO_{2} after 10year injection. As could be seen from Figure
Boxwhisker plots of sequestered CO_{2} in aquifer after 10year injection: Cases A, B, and C.
Figure
Boxwhisker plots of sequestered CO_{2} in aquifer after 10year injection for all considered cases (
In Figure
Vertical displacement of top of caprock: Cases A, B, and C.
Figure
Boxwhisker plots of vertical displacement: Cases A, B, and C.
The amounts of sequestered CO_{2} and range of uncertainty in vertical displacement are larger for models with higher Dykstra–Parsons coefficient. It indicates that heterogeneity for petrophysical properties result in unfavorable effects in flow output variables. In contrast, the heterogeneity of rock mechanical properties can reduce additional escape of stored CO_{2}. In terms of geomechanical change, heterogeneities of both petrophysical and geomechanical properties can affect the uplift of caprock. Highly heterogeneous caprock will be transformed more. Despite the heterogeneity of geomechanics affects the displacement of caprock, Case B shows relatively small range of uncertainty. It is obvious that geomechanical properties affect mechanical change of formation. However, there is limitation of displacement when reservoir properties are correlated. Since both geomechanical and flow responses are of importance to predict and optimize CO_{2} injection performance, ignoring heterogeneity effects for storage site properties may result in inaccurate analyses.
In order to assess the effect of tectonic stresses, horizontal stress is added to highly heterogeneous model (c) (
Cumulative inplace amounts of CO_{2} in aquifer at different times under the additional tectonic stress (
Vertical displacement of top of caprock under the additional tectonic stress (
Comparisons among storage sites with heterogeneous caprock showed the significance of coupled simulation of fluid flow and geomechanical deformation with respect to CO_{2} storage integrity. In particular, they have shown the importance of correlation between hydraulic and geomechanical properties in controlling CO_{2} leakage through caprock. Four major factors affecting mechanical caprock failure considered in this study are permeability, porosity, Young’s modulus, and Poisson’s ratio. These four properties are correlated as linear relationship based on field data. Heterogeneous petrophysical properties have unfavorable effects on CO_{2} storage, and geomechanical heterogeneity could mitigate further escape of CO_{2} after leakage occurs. Heterogeneity effects of these two properties can be offset for CO_{2} trapping. Heterogeneity of these properties also affects on vertical displacement of caprock. Results of analysis show that heterogeneity of four properties induces further displacement of caprock, but geomechanical properties have limited effects when it is correlated with other properties as a function. Subsurface storage needs to be evaluated in terms of heterogeneity to avoid any risk when operating CO_{2} sequestration. The hydrogeomechanical correlations are applied for more accurate investigation of CO_{2} leakage. We considered only four properties because correlations between hydraulic properties and other geomechanical properties have not been thoroughly understood. However, investigation including other formation properties such as rock compressibility and yield stress could provide better understanding of geomechanical study. Another improvement on the analysis of caprock stability can be achieved by the evaluation of 3D model. It could be an effective way to evaluate more realistic CO_{2} flow regime.
The authors declare that they have no conflicts of interest.
This work was supported by the Energy Efficiency & Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (no. 20152520100760).